This invention relates to the field of magnetic resonance imaging (MRI), where radiofrequency (RF) magnetic fields are used to interrogate a region of interest. It is particularly directed towards the shielding of RF effects which occur from inside and outside of the MRI system.
One of the components in MRI systems is an RF coil, which is part of a typical MRI data gathering sequence. MRI RF signals of suitable frequencies are transmitted into the imaging volume and NMR responsive RF signals are then received from the imaging volume via one or more RF coils or antennae. Information encoded within the frequency and phase parameters of the received RF signals is then processed to form visual images representing the distribution of NMR nuclei within a cross-section or volume of the patient within the volume of interest (VOI) of the MRI system.
In MRI devices (MRDs) which are meant for imaging body extremities, only the portion to be examined in the MRI is located within the bore of the MRI system, whereas the rest of the body remains outside of the MRI. In such cases, RF signals from the volume outside the MRI may be collected through the conductive body and the extremity to be examined, having dielectric properties, may serve as a transceiver of RF signals, thus causing noise in signal collection during an MRI examination.
Additionally, RF fields within the MRI can induce an electrical current in the body which is transformed into energy in the form of heat. Heating of tissues is due to resistance in the tissues is called “ohmic heating”. Specific Absorption Rate (SAR) is key variable in determining patient heating potential in an MR scanner is the RF power absorbed by the body per unit mass, generally measured in the unit of W/kg. If the SAR exceeds the thermal regulation capacity the patient's body temperature will rise.
U.S. Pat. No. 5,304,932 is directed towards shielding an MRI RF coil from extraneous noise sources using an extremely thin conductive shield interposed between the RF coil and the static magnetic structure of an MRI system. To control eddy currents induced in such a conductor by the changing magnetic flux of MRI gradient coils, the RF shield conductor thickness is less than three skin depths at the MRI RF operating frequencies of the RF coil. Preferably, the RF shield conductor thickness is on the order of only one skin depth or less.
U.S. Pat. No. 7,801,613 is directed towards housing of implantable medical devices in a titanium alloy that provides improved electrical performance, mechanical strength, and reduced MRI heating.
There thus remains a long felt need for reducing the electromagnetic energy propagation from MRI's magnet bore to the outer environment surrounding the magnet and vice versa, for a protecting means providing effective Faraday shielding used as a barrier between the internal and external RF fields, and for reducing SAR of a non-examined portion of a body extremity in the vicinity of a MRD.